Polishing technology has been largely driven by the need for exceptionally smooth and planarized surfaces on high-tech materials and components such as magnetic disks, semiconductors. and the like. In the case of semiconductor wafers, for example, polishling, techniques are employed, not just for polishing and planarizing the bulk wafer, but for planarization of those layers which comprise the active circuitry; e.g., conductor metals. passivation, and interlayer dielectrics.
In a paradigmatic polishing operation, a platen/polishing-pad assembly is employed in conjullction with a slurry, for example a water-based slurry comprising colloidal silica particles. When pressure is applied between the polishing pad (e.g., a polyurethane pad) and the workpiece being polished. mechanical stresses are impaited to the workpiece surface. Abrasive particles within the slurry act to create zones of localized stress. which in turn creates mechanical strain on the chemical bonds comprising the surface being polished. Consequently, microscopic regions are removed from the surface being polished, enhancing planarity of the polished surface.
Furthermore, in a chemical-mechaanical planarization context (CMP), the slurry is used to effect chemical as well as mechanical polishing and planarization. More particularly, the slurry suitably comprises a chemically and mechanically active solution, for example, abrasive particles coupled with chemically reactive agents. Suitable chemically reactive agents include hydroxides but may also include highly basic or highly acidic ions.
See, for example, Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud, U.S. Pat. No. 5,498.196, issued March, 1996; Arri, et al., U.S. Pat. No. 4,805,348, issued February, 1989; Karlsrud et al., U.S. Pat. No. 5,329,732, issued July, 1994; and Karlsrud et al., U.S. Pat. No. 5,498,199, issued March, 1996, for further discussion of presently known lapping and planarization techniques. By this reference. the entire disclosures of the foregoing patents are hereby incorporated herein.
A key variable used to characterize a particular polishing or grinding process is the material removal rate. The material removal rate of a process is simply the rate at which material is removed from the workpiece surface, and is typically expressed as a length prr unit time (e.g.. microns per minute).
Many factors can and do affect material removal rate. For example, the material properties of the polishing surface, the mechanical and chemical properties of the slurry, and the properties of the workpiece surface itself are all important factors. In addition, and more important for the purposes of the present invention, removal rate is a strong function of applied pressure. That is, removal rate increases as the local normal compressive force applied to the workpiece surface increases.
Presently known polishing/grinding techniques are unsatisfactory in several regards. In many polishing configurations, for example, particularly where processing of multiple or large workpieces is performed, material removal rate can vary significantly from workpiece-to-workpiece and across individual workpieces themselves due to plate misalignment. More particularly. even when optimum plate alignment is achieved during initial setup, small changes in polishing machine position and orientation can have a significant impact on plate alignment due to a lack of robustness in presently known stabilization techniques. Such changes might occur, for example, due to movement of the building in which the polishing machine resides, or larger scale changes due to seismic shifts.
Presently known techniques are also unsatisfactory in that, over time, day-to-day operation of a typical polishing machine can also result in plate misalignment. Tllis misalignment can be remedied by repeated realignments pursuant to a preventive maintenance schedule; however, such realignments can be quite frequent, resulting in substantial machine down-time.
Polishing techniques are thus needed which provide improved plate stabilization, thus reducing variations in workpiece pressure during polishing.